This invention relates generally to integrated circuit packaging and more particularly to lead-frame type integrated circuit packaging.
Integrated circuit chips (or "dies") can be packaged in a variety of ways. For example, pre-manufactured I.C. packages are made from plastic, ceramic, or other suitable materials which include a body, a number of conductive leads emanating from the body, and a die cavity. An I.C. die is attached within the die cavity and is wired to the leads of the package with a multiplicity of connecting wires. A lid is then placed over the die cavity to hermetically seal the I.C. die within the pre-manufactured package.
Another common packaging method utilizes a lead frame including an I.C. die support and a number of conductive fingers extending generally radially away from the support. An I.C. die is attached to the support and is wired to the fingers with a number of connecting wires. The lead frame and wired I.C. die are then placed within the cavity of a transfer molding apparatus and a plastic material is injected into the cavity to encapsulate the I.C. die and much of the lead frame. The ends of the lead frame fingers are then cut and bent to form the leads of the packaged integrated circuit.
The number of input/output (I/O) attach pads on an integrated circuit die tends to increase with increasing complexity of the integrated circuitry. For example, large scale integration (LSI) will have on the order of 64 attach pads on an I.C. die, while the current generation of very large scale integration (VLSI) will have on the order of 160 attach pads on an I.C. die. The next generation of ultra large scale integration (ULSI) may have 300 or more attach pads on a single integrated circuit die.
The large number of attach pads on a modern I.C. die can result in difficulties with the connecting wires of lead-frame type packages. The wires, which are often 1-2 thousandths of an inch (Mils) in diameter are very flimsy and can easily short out with adjacent wires if they are even slightly displaced due to vibration, impact, etc. This problem is compounded by the fact that the bulk material for the connecting wires is purchased as a tightly wound spool of wire, which results in the connecting wires exhibiting an inherent twist. If two adjacent connecting wires twist in opposite directions the chance of a short is greatly increased. Furthermore, with lead frame type packages the injection of the plastic compound to encapsulate the I.C. die can cause the wires to flex in the direction of fluid flow (a phenomenon known as "sweep"), thereby increasing the likelihood of shorting out adjacent wires. Therefore, lead frame type packages are much more vulnerable to shorting of the connecting wires than pre-manufactured plastic or ceramic packages.
One possible solution to this problem is to insulate the connecting wires so that if they do touch they don't short out. Another solution is to shorten the connecting wires by use of an interposer. However, these solutions are difficult to implement and tend to be quite expensive, and therefore are not used much within the integrated circuit industry.
There is also a reverse correlation between integrated circuit complexity and I.C. die size. Modern I.C. die tend to be smaller and more complex and therefore require longer connecting wires. These longer wires greatly increase the shorting problem between adjacent wires. One solution to the long connecting wire problem is to reduce the size of the pre-manufactured packages or lead-frame type packages. However, this solution is not desirable for several reasons. For pre-manufactured packages, it is more cost-effective and practical to standardize on a few package sizes which accommodate both large and small die sizes. For lead-frame type packages, it gets very expensive to decrease the size of the I.C. support and to bring the fingers in closer to the support. This is because as the fingers get smaller and closer together it is no longer possible to inexpensively stamp out the lead frames from a metal foil and, instead, requires that the lead frames be made by a much more expensive metal-foil etching process. Furthermore, even with metal-foil etching processes there are severe technical limitations on how closely the lead fingers can be spaced for a given foil thickness.
Another problem with smaller die sizes having large numbers of attach pads is how to position the tip of a wire bonding tool on a bonding pad without contacting and possibly damaging adjacent bonding pads and/or wires. One solution to this problem is to stagger the attach pads in a zig-zag fashion to provide larger spaces between adjacent pads while simultaneously increasing the effective pitch of the attach pads. Another solution used in pre-manufactured ceramic packages is to have several shelves within the die cavity having contacts leading to the leads of the package. Of necessity, the connecting wires in this type of pre-manufactured package are of varying lengths and possibly loop heights depending upon which shelf to which they are attached. This multishelf ceramic package solution was devised to address the aforementioned bonding tool tip access problem and not the connecting wire shorting problem.
Modern VLSI circuitry is predominantly packaged with lead frame packaging for cost considerations. As such, virtually all of the factors leading to shorts between connecting wires are present. It is economically undesirable to create lead frames with fingers which extend close to the small I.C. dies for the reason mentioned above. This necessitates rather long connecting wires which, due to their inherent twist and due to injection molding sweep, have a good chance of contacting adjacent wires. Insulating the connecting wires tends to be economically impractical. The connecting wires could be made thicker, but this would necessitate bigger attach pads on the I.C. dies and a larger pitch between the attach pads, both of which are undesirable because they increase the size of the I.C. dies. In consequence, the prior art has not adequately addressed the problem of how to economically package a small integrated circuit die having long connecting wires in a lead frame type package without shorting out adjacent connecting wires in an unacceptably large number of I.C. assemblies.